Color laser printers utilize an electrophotographic process for recording and registering a multi-color image on an electrophotographic surface or a print medium, such as paper. Image data representing each primary color, such as those generated in a personal computer, are sent to the laser printer, which converts the image data to electrical signals that represent dots forming the image. Individual electrical signals modulate a laser beam as it is directed to an electrically charged, light-sensitive surface of a drum, which records the image pattern. The laser beam strikes and electrically discharges areas of the light sensitive drum to produce a latent electrostatic image on the drum surface. After it is scanned, the drum surface is positioned near a developing toner source, which is typically an electrically biased roller, to repel toner and thereby transferred to the drum surface to form a pattern that represents the image. The toner can be transported to the drum surface by dry carrier beads or suspended in a liquid vehicle. The toner is then transferred in accordance with an electrostatic process from the drum surface to the print medium.
Toner is a mixture of pigment and plastic (resin). A multi-color laser printer develops successive images by using developing toners of different colors supplied from corresponding toner sources. Color printing is typically done with the subtractive primary colors and with black. All of these colors are applied in registration during successive rotations of the drum before transfer of the toner to the print medium or the colors are applied to the print medium in registration by a series of in-line drums. Heat is applied (by passing the medium through a heater fuser) to permanently fuse the image to the print medium to form a finished multi-color image.
Laser driven color printers and copiers employ transparent toners which enable light to reflect off the page and to be directed back towards the eye. In general, such devices employ cyan (C), magenta (M) and yellow (Y) toners as the principal component colors, from which other colors are created. Light passing through CMY toners has part of its color filtered out or absorbed by the toner such that the reflected light takes on the color of the toners that it passes through. In laser printers (and some copiers), a black (K) toner is used which is opaque to light. Overall, more color and darker blacks mean a more photographic look to images. However, if a K toner is overprinted onto CMY to achieve darker colors, such as found in shadows, much of the colorfulness of the shadows is typically lost.
Alternatively, the prior art has employed combinations of the three primary colors CMY to produce darker colors ranging to black. Prior art printing procedures have also utilized combinations of K and CMY toners to achieve dark or shadowed color images. The need to deposit 100% density toner layers for each color (and, in some cases, black) typically causes an excessive amount of toner to be deposited on the media sheet. Such a high level of toner deposition does not fuse well and, in general, creates unsatisfactory images. Merely increasing the fusing time or temperature is not always feasible because of the differences in toners, media types, or excess heat that exists during fusing of the second side of a duplex page. Additionally, overfusing can cause media to curl, warp or jam the printer.
It would be desirable to provide toners that have improved chroma and darkness when printed on various media, while maintaining or improving the fusing, scattering, duplexing characteristics of the toners.